Stability and Controls

Basic Controls

page 1

As mentioned earlier, the primary controls the pilot uses are the ailerons, the elevators, and the rudder. These control surfaces are controlled by the pilot to maneuver the airplane. The elevators are flaps on the horizontal stabilizer (the horizontal part of the tail assembly). The rudder is the moveable flap on the vertical stabilizer of the tail. The ailerons are the flaps along the outer portions of the trailing edges of the wings. These are shown clearly in the figure below.

To make a turn, the pilot will move a controlling device in the cockpit, usually either a stick in fighter-type aircraft or a yoke in most others. This action deflects one aileron upward and the other aileron downward. This increases the lift on the wing with the aileron deflected downward, and it decreases the lift on the other side with the aileron deflected upward. The airplane will roll with the high lift wing up and the lower lift wing down and begin to turn. If a pilot wishes to turn right, he or she will raise the right aileron and lower the left aileron (by moving the stick to the right or turning the yoke to the right). The right wing will dip as the left wing rotates upward. Rolling the airplane is considered a lateral motion, so the ailerons are called lateral controls.

The pilot uses the elevators on the tail to pitch the airplane. If he or she pulls back on the stick/yoke, the elevators deflect up, the lift on the tail surfaces decreases, and the nose will pitch up. This will increase the angle of attack of the airplane with respect to the airflow and increase the overall lift on the plane. As discussed in the previous section, the altitude of the plane will increase. A pitching motion is a longitudinal motion, and the elevators are known as longitudinal controls.

To induce a yawing motion, the pilot will use the rudder. Almost all rudders are controlled by pedals operated by the feet of the pilot. If the pilot steps on the right rudder pedal, the rudder is deflected to the pilot's right and a clockwise rotation about the z axis results. The pressures on the right side of the rudder are higher than the pressures on the left side, so the aerodynamic force on the rudder goes to the left. This causes the plane to rotate clockwise. The rudder is called a directional control, since yawing is considered a directional motion.

Rolling and yawing of the airplane are related; it is very hard to do one without the other. During a right turn, for example, when the lift is increased on the left wing, drag is also increased on that wing. The result is a small rotation around the z-axis to the left. This condition is called adverse yaw, and the pilot corrects for it by applying a small amount of right rudder. So, in most cases he or she will use both the ailerons and the rudder to turn the aircraft.

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Last modified: Sun Aug 2 12:04:44 PDT 1998

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